1. Field of the Invention
The present invention relates to integrated circuits which include at least one memory having redundancy elements. The invention is more particularly concerned with elements for switching-over to the redundancy elements of the memory.
2. Description of the Prior Art
Redundancy elements are components which are substituted for faulty elements of the memory. The substitution must be transparent for the memory user. When a user addresses a memory cell and when this cell has proved to be defective at the time of testing, a circuit has memorized this fault condition so as to initiate a change over to the desired redundancy element which is consequently substituted for the defective cell without the user's knowledge. However, in order to perform a switching operation of this type, it is necessary to provide memory cells which are dedicated to storage of information for switching to a predetermined redundancy cell in respect of each faulty cell.
This storage operation takes place at the time of the first memory test when the integrated circuit is mounted on a test board prior to encapsulation. Storage is performed by blowout or non-blowout of fuses provided for this purpose. The desired redundancy element can be addressed by first determining the state of these fuses, this being achieved by measuring the voltage between one terminal and a reference potential.
In circuits of the MOS type, low-value resistors provided in the form of a polycrystalline silicon layer are accordingly "broken-down" by means of a high electric (or laser) discharge at the time of testing of the memory. The polycrystalline silicon layer disappears and the resistor has an infinite impedance. A detection circuit is provided for each fuse in order to determine its state. The disadvantage of this method lies in the need to provide a detection circuit for each fuse.
Furthermore, in order to blow a fuse, it is necessary to supply a high voltage which is not employed in memories as a general rule since the voltages required for the read operation, for example, are between 0 and 5 volts.
The only high voltage available at the present time is the programming voltage Vpp which is selected by the customer. Requisite programming voltages are in the vicinity of 10 to 15 volts. In point of fact, the development of technology is attended by a downward trend in required values of programming voltage. The result is that, when the technology has reached a sufficiently advanced stage of development, it will prove impossible to use any memory programming voltage for blowout of fuses reserved for the switching circuit.
In the present state of the art, it is a customary practice to utilize this programming voltage Vpp for blowing these fuses. To this end, the voltage is applied from a common terminal which is used for programming of the memory and also for fuse blowout when carrying out tests. The fuses are connected directly between the potential Vpp and ground via a control transistor. Blowout of fuses takes place when the transistors are triggered into conduction.
This solution is not without danger. In fact, since the supply voltage is common to fuse blowout and to programming, the fuses which have not blown are liable to receive a potential difference since the control transistors, even if they are not turned-on, are nevertheless put in a conductive state by a punch-through phenomenon which occurs in the presence of a high voltage.
The characteristics of fuses therefore suffer degradation during programming of memories. This aging process eventually leads to an unintentional change in the state of the fuses.
The present invention proposes to overcome the drawbacks outlined in the foregoing and to provide in the integrated circuit of the memory type a supplementary terminal which is connected to all the fuses but is not intended to be connected to the external pins of the integrated circuit. Said supplementary terminal is connected to a circuit which generates a low voltage at the time of programming of the memory and which is capable of presenting high impedance when a high voltage is applied to said supplementary terminal in order to ensure that the entire current flows through the fuses selected for blowout.